Epithelia form boundaries of biological compartments, creating specialized absorptive and secretive surfaces such as the kidney tubules, the intestinal tract, and the mammary gland. The ability of epithelial cells to regulate absorption and secretion of essential ions such as sodium, chloride, calcium, and magnesium is critical for the maintenance of electrolyte balance (Van Itallie and Anderson, 2006). Ion transport across an epithelial layer can be either transcellular or paracellular (Shen et al., 2011). The transcellular pathway involves the movement of ions across the cytoplasm via plasma membrane channels, carriers, and exchangers (Muto et al., 2011). The paracellular pathway involves the movement of ions through the intercellular spaces between epithelial cells. The transmembrane proteins of tight junctions include claudins, junctional adhesion molecules (JAMs), occludin and tricellulin. Chiba et al. (2008) have provided an overview of these proteins, highlighting their roles and regulation, as well as their functional significance in human diseases. Sequence analysis of claudins has led to differentiation into two
groups, designated as classic claudins (1-10, 14, 15, 17, 19) and
non-classic claudins (11-13, 16, 18, 20-24), according to their degree
of sequence similarity (Krause et al., 2008). Claudins have been reviewed from structural/functional standpoints (Krause et al. 2015). All of the identified tight junction transmembrane proteins can be multiply phosphorylated, but only in a few cases are the consequences of phosphorylation at specific sites well characterized (Van Itallie and Anderson 2017).

The architecture of tight junctions can be conceptualized into
compartments with the transmembrane barrier proteins (claudins, occludin, JAM-A, etc.), linked to
peripheral scaffolding proteins (such as ZO-1, afadin, MAGI1, etc.) which are in turned linked to
actin and microtubules through numerous linkers (cingulin, myosins, protein 4.1, etc.) (Van Itallie and Anderson 2014). Within this
complex network are associated many signaling proteins that affect the barrier and broader cell
functions. The PDZ domain is a commonly used motif to specifically link individual junction protein
pairs. Van Itallie and Anderson 2014 reviewed some of the key proteins defining the tight junction as well as their detailed architecture and subcompartments. Claudins 1 and 3 can form homo- and heterophilic cis and trans interactions, and at least two different cis-interaction interfaces within claudin-3 homopolymers as well as within claudin-1/claudin-3
heteropolymers have been documented (Milatz et al. 2015).

Two TJ protein families can be distinguished, claudins, comprising 27
members in mammals, and TJ-associated MARVEL proteins (TAMP), comprising
occludin, tricellulin,
and MarvelD3 (Krug et al. 2014). They are linked to a multitude of TJ-associated
regulatory and scaffolding proteins. The major TJ proteins are
classified according to the physiological role they play in enabling or
preventing paracellular transport. Many TJ proteins have sealing functions (claudins 1, 3, 5, 11, 14, 19, and tricellulin).
In contrast, a significant number of claudins form channels across TJs
which feature selectivity for cations (claudins 2, 10b, and 15), anions
(claudin-10a and -17), or are permeable to water (claudin-2). For
several TJ proteins, function is yet unclear as their effects on
epithelial barriers are inconsistent (claudins 4, 7, 8, 16, and
occludin). TJs undergo physiological and pathophysiological regulation
by altering protein composition or abundance. Major pathophysiological
conditions which involve changes in TJ protein composition are (1)
effects of pathogens binding to TJ proteins, (2) altered TJ protein
composition during inflammation and infection, and (3) altered TJ
protein expression in cancers (Krug et al. 2014).

The gatekeeper of the paracellular pathway is the tight junction, which is located at apical cell-cell interactions of adjacent epithelial cells. Three known inherited disorders, familial hypomagnesemia (Simon et al., 1999), hypertension (Wilson et al., 2001), and autosomal recessive deafness (Wilcox et al. 2001) have been linked to proteins that localize at the tight junction. Transmembrane proteins of tight junctions include claudins, junctional adhesion molecules (JAMS), occludin and tricellulin. The cytoplasmic scaffolding proteins include Z0-1, -2 and -3 (Hartsock and Nelson, 2008).Their study has led to insights into the molecular nature of tight junctions (Chiba et al., 2008). Neurological diseases (Bednarczyk and Lukasiuk, 2011) and renal diseases (Li et al., 2011) have been reviewed. High concentrations (>200 μM) of Zn2+ can affect TJ integrity in a polarized manner. Thus, the basolateral addition of Zn2+ leads to reversible TJ opening with pore paths of r ∼ 2 nm or more, depending on the Zn2+ concentration. Zn2+-induced paracelluar channels favour efflux especially for macromolecules (Xiao et al. 2018).

Tight junctions of epithelial cells exclude macromolecules but allows permeation of ions. It has not been clear whether this ion-conducting property is mediated by aqueous pores or by ion channels. To investigate the permeability properties of the tight junction, Tang & Goodenough (2003) developed paracellular ion flux assays for four major extracellular ions, Na+, Cl-, Ca2+, and Mg2+. Tight junctions share biophysical properties with conventional ion channels, including size and charge selectivity, dependency of permeability on ion concentration, competition between permeant molecules, anomalous mole-fraction effects, and sensitivity to pH. Their results support the hypothesis that discrete ion channels are present at the tight junction. Unlike conventional ion channels, which mediate ion transport across lipid bilayers, the tight junction channels must orient parallel to the plane of the plasma membranes to support paracellular ion movements. This new class of paracellular-tight junction channels facilitates the transport of ions between separate extracellular compartments (Balkovetz, 2009). Claudin-2 forms highly cation-selective paracellular pores (Yu, 2009). The basis
of this charge selectivity is likely to be the presence of a negatively
charged binding site within the lumen of the pore. Paracellin-1 may be a Mg2+ transporter (Brandao et al. 2012).

Heterotypic (head-to-head) binding between different claudin isoforms plays a role in regulating paracellular permeability. Claudin-3 and claudin-4 do not heterotypically interact despite having highly conserved extracellular loop (EL) domains (Daugherty et al., 2007). Claudin-1 and -5, which are heterotypically compatible with claudin-3, do not bind to claudin-4. In contrast, claudin-4 chimeras containing either the first EL domain or the second EL domain of claudin-3, do bind to claudin-1, claudin-3, and claudin-5. Moreover, a single point mutation in the first extracellular loop domain of claudin-3, converting Asn44 to the corresponding amino acid in claudin-4 (Thr) produced a claudin capable of heterotypic binding to claudin-4 while still retaining the ability to bind to claudin-1 and -5. Thus, control of heterotypic claudin-claudin interactions is sensitive to small changes in the EL domains (Daugherty et al., 2007).

Claudins comprise the primary constituents of tight junctions and determine paracellular permeability. Ion selectivity of the paracellular conductance is a complex function of claudin subtype and cellular context (Hou et al., 2007). These 4 TMS proteins have been characterized from structural standpoints and may have arisen from an early intragenic duplication event (Hua et al., 2003). There are 27 claudin paralogues in mice and humans (Mineta et al. 2011). Permselective
paracellular claudin channels are specific for certain ions and non-ionic solutes. Recent studies
using claudin knockout mice revealed that the loss of claudins' specific paracellular barrier and/or
channel functions affects particular biological functions and leads to pathological states (Tamura and Tsukita 2014).

As reviewed by Angelow et al. (2007;2008), the structure of claudin-based paracellular pores is largely unknown, but it is probably composed of homo- and hetero-typic claudin digomers. Both the proteins involved and the cell type determine the selectivity of paracellular transport. Claudins 2, 106 and 15 act preferentially as cation pores while claudins 10a and 7 are the only claudins that have significant anion pore properties (Angelow et al., 2008). However, claudins 4 and 7 have been reported to act as cation pores in MDCK II cells but as anion pores in LLC-PK 1 cells (Hou et al., 2006). They can pass neutral as well as charged small molecules. Their pore diameters are 8-15 Å. The first extracellular loop may line the paracellular pathways and determine the charge selectivity, but the C-terminal tail, which is modified by phosphorylation and palmitoylation and interacts with cytoskeletal proteins, may also play a role.

Claudin-2 pores are narrow, fluid filled, and cation
selective (Yu et al., 2009). Charge selectivity is mediated by the electrostatic
interaction of partially dehydrated permeating cations with a
negatively charged site within the pore that is formed by the side
chain carboxyl group of aspartate-65. Thus, paracellular pores use
intrapore electrostatic binding sites to achieve a high conductance
with a high degree of charge selectivity.

The control of claudin assembly into tight junctions requires a complex interplay between several classes of claudins, other transmembrane proteins and scaffold proteins (Findley and Koval, 2009). Claudins are also subject to regulation by post-translational modifications including phosphorylation and palmitoylation. Several human diseases have been linked to claudin mutations. Roles for claudins in regulating cell phenotype and growth control suggest a multifaceted role for claudins in regulation of cells beyond serving as a simple structural element of tight junctions.

Epithelial transport relies on the proper function and regulation of the tight junction (TJ); otherwise, uncontrolled paracellular leakage of solutes and water would occur. They also act as a fence
against mixing of membrane proteins of the apical and basolateral side. The proteins determining
paracellular transport consist of four transmembrane regions, intracellular N and C terminals, one
intracellular and two extracellular loops (ECLs). The ECLs interact laterally and with counterparts
of the neighboring cell and thereby achieve a general sealing function. Two TJ protein families can
be distinguished, claudins, comprising 27 members in mammals, and TJ-associated MARVEL proteins
(TAMP), comprising occludin, tricellulin, and MarvelD3. They are linked to a multitude of TJ-
associated regulatory and scaffolding proteins (Günzel and Fromm 2012). The major TJ proteins are classified according to
the physiological role they play in enabling or preventing paracellular transport. Many TJ proteins
have sealing functions (claudins 1, 3, 5, 11, 14, 19, and tricellulin). In contrast, a significant
number of claudins form channels across TJs which feature selectivity for cations (claudins 2, 10b,
and 15), anions (claudin-10a and -17), or are permeable to water (claudin-2). For several TJ
proteins, their functions are unclear as their effects on epithelial barriers are inconsistent (claudins
4, 7, 8, 16, and occludin). TJs undergo physiological and pathophysiological regulation by altering
protein composition or abundance. Major pathophysiological conditions which involve changes in TJ
protein composition are (1) effects of pathogens binding to TJ proteins, (2) altered TJ protein
composition during inflammation and infection, and (3) altered TJ protein expression in cancers (Günzel and Fromm 2012).

The electric property of claudin
pertains to two important organ functions: the renal and sensorineural functions. The kidney
consists of three major segments of epithelial tubules with different paracellular permeabilities:
the proximal tubule (PT), the thick acending limb of Henle's loop (TALH) and the collecting duct
(CD). Claudins act as ion channels allowing selective permeation of Na+ in the PT, Ca2+ and Mg2+ in
the TALH and Cl- in the CD. The inner ear, on the other hand, expresses claudins as a barrier to
block K+ permeation between endolymph and perilymph. The permeability properties of claudins in
different organs can be attributed to claudin interactions within the cell membrane and between
neighboring cells. The first extracellular loop of claudins contains determinants of paracellular
ionic permeability (Hou 2013).

The thick ascending limb (TAL) of Henle's loop drives paracellular Na+, Ca2+, and Mg2+ reabsorption
via the tight junction (TJ). The TJ is composed of claudins with two extracellular segments (ECS1 and -2), and one intracellular loop. Claudins interact
within the same (cis) and opposing (trans) plasma membranes. Claudins Cldn10b, -16, and -19
facilitate cation reabsorption in the TAL, and their absence leads to disturbances of renal
ion homeostasis. Milatz et al. 2017 showed that (i) TAL TJs show a
mosaic expression pattern of either cldn10b or cldn3/cldn16/cldn19 in a complex; (ii) TJs dominated
by cldn10b prefer Na+ over Mg2+, whereas TJs dominated by Cldn16 favor Mg2+ over Na+; (iii) Cldn10b
does not interact with other TAL claudins, whereas Cldn3 and Cldn16 can interact with Cldn19 to form
joint strands; and (iv) further claudin segments in addition to ECS2 are crucial for trans
interaction. Milatz et al. 2017 suggested the existence of at least two spatially distinct types of paracellular
channels in TAL: a Cldn10b-based channel for monovalent cations such as Na+ and a spatially distinct
site for reabsorption of divalent cations such as Ca2+ and Mg2+.

The paracellular transport reactions proposed to be catalyzed by claudinins are: